This invention relates generally to a system for projecting an image of a document lying in an object plane onto a photosensitive image plane and, more particularly, to an improved system adapted to correct for cos.sup.4 illumination falloff and other illumination nonuniformities so as to provide a uniform illumination exposure level at the image plane.
In imaging systems such as those utilized in photocopiers, a document supported on a transparent surface is scanned by an illumination scan assembly, or is flash illuminated, and the document image is projected through a lens onto the charged surface of a photoreceptor. Those areas of the photoreceptor which are irradiated by the projected image are discharged, the degree of discharge being dependent upon the intensity of the impinging light rays. It is important that the exposure irradiance at the photoreceptor vary only due to the reflectance characteristics of the original document being copied rather than due to changes introduced by the imaging components. Stated in another manner, the optimum system would be one providing uniform photoreceptor irradiance given a uniformly reflecting documents.
Of the factors affecting relative illumination at an image plane, the most well known is the cosine (cos) variation caused by the projection lens wherein the illuminaton at the image plane is proportional to the cos.sup.4 of the angle between the optical axis and the field beam. (A discussion of this phenomenon is provided in Rudolf Kingslake's "Lenses in Photography, pp. 100, 101 (Garden City Books 1951). In a photocopier, phtoreceptor irradiance decreases as radial distance from the system optical axis increases. Various approaches have been devised to compensate for this effect. Typically, in slit-scanning systems, a sheet of opaque material having a butterfly slit formed thereon is placed in the optical path near the object or image plane acting as a field stop. The area of the slit is inversely proportional to the illumination profile. Other similar techniques employ masks having slits of this nature integral with the lens. Still other scanning systems place a variable density filter in the optical path, the transmissiveness of the filter varying inversely to the cos.sup.4.
It is also known to use optical stops in photographic systems to reduce oblique spherical aberrations. A system employing a sagittal stop is disclosed in U.S. Pat. No. 3,504,960. This type of stop is characterized by having borders of various configurations surrounding a central clear aperture. These stops are not, however, concerned with, and are not substitutes for, cos.sup.4 nonuniformity compensation.
In full frame imaging systems wherein an entire document is typically illuminated by flask illumination, efforts to compensate for cos.sup.4 light falloff have emphasized locating light sources in such a way that the document edges are illuminated to a greater degree than central areas. Two such systems are disclosed in U.S. Pat. Nos. 3,669,538 (Fowler) and 3,777,135 (Rees). Another technique is to place a variable density filter technique into the optical path as disclosed in U.S. Pat. Nos. 3,097,255 and 4,298,274.
The above-described compensation techniques each have significant disadvantages. The variable density filters are relatively expensive and entail significant energy losses. The optical slit techniques involve energy losses and also require substantial modification of the imaging system so as to operate in more than one magnification reproduction mode. The present invention is therefore directed to an illumination corrector which is inexpensive, easily placed into the optical path and adapted to provide the required cos.sup.4 compensation throughout a magnification range.
In addition to cos.sup.4 corrections, the illumination corrector can compensate for variations of document irradiance if the variations are well defined and symmetrical with respect to field position. The blocker can also compensate for illumination variations as evident by pupil distortion which is common in wide angle lenses.